The invention relates to a data acquisition network formed of nodes and in which some nodes exchange data by wireless link, other nodes exchanging data by wire or cabled link.
The invention relates in particular to a network in which the nodes are formed of seismic sensors.
The invention also relates to a seismic sensor comprising means for sending and receiving data by wireless link and/or by wire link.
The present invention applies in particular in the field of oil exploration. In this field of application, seismic sensors, disposed on the earth's surface, are used to receive the echoes of waves emitted into the ground by a seismic source. A processing of these echoes makes it possible to construct a map of the explored sub-surface.
The invention is not however limited to the particular field of oil exploration and may be applied to any field implementing a data acquisition network in which the data acquired by each of the nodes of the network are sent and received by other nodes by wireless link and/or by wire link.
The person skilled in the art is aware of numerous devices implementing a data acquisition network.
In the case where the data acquisition network is formed of an assembly of seismic sensors, these devices are conventionally and usually constituted by sensors linked together in groups to a “node” of the network, by means of wires or cables. These nodes are generally organized in groups around a “concentrator”, a group of nodes forms, with its concentrator, a subnetwork, the links between the concentrator and the nodes likewise being wire links. The concentrators locally manage the subnetwork of nodes, provide the energy supply for the nodes and centralize the data. The concentrators are connected to a local computer network, likewise a wire network, to which is connected a Central Unit which drives the assembly and records the data originating from the subnetworks.
These conventional solutions are well known to the person skilled in the art and will not be described in detail hereinbelow. These solutions have the drawback of requiring a significant quantity of cables and associated lengths of cables, cables difficult to move and to deploy on an operating site.
Networks for acquiring data by wireless link (RF) have therefore been proposed, in particular for seismic applications, since they have numerous advantages. Wireless networks of seismic sensors are networks whose nodes are equipped with at least one sensor. The data acquired at the level of each of the nodes are transmitted at low power so as to reach at least one neighboring node, which serves as relay to reach another neighboring node and so on and so forth until the data reach their destination, which is generally the concentrator. This technique is called the multi-hop technique.
The wireless communication from one node to another of the network can in certain cases be difficult or even impossible.
By way of nonlimiting examples, the following causes for which wireless communication between two nodes of the network is difficult or even impossible may be cited:                Multipaths: the RF waves are reflected off the surrounding obstacles and the antenna of a sensor of the receiver node may be located at a spot where the sum of the RF waves is close to zero;        The topology of the terrain: a large difference in altitude between two neighboring nodes of the network may mask the RF beam (canyon, cliff, etc.);        Vegetation: bushes, dense areas of plants;        Damp, flooded or floodable areas (the RF waves do not propagate in water): marshes, areas prone to tides;        Buried sensors (happens in some cases): a buried antenna is very insensitive;        Jammers: the proximity of transmitters in the same frequency bands (same radio band) or in nearby bands may render the radio messages sent and received between two nodes of the network totally undecodable.        
The nodes of these wireless networks are customarily equipped with a single antenna used alternately for receiving and sending the data acquired. With a single antenna, the data transmission problems cited herein-above are even more acute.
Document FR 2511772 proposes that this type of data transmission problem be solved with a device performing the sequential transmission of the data between a central system for control and recording of the data (concentrator) and a plurality of data acquisition apparatuses (seismic sensors forming a node) by wireless transmission (RF waves) and by wire transmission (cables).
Thus, in the areas where the transmission of the data by RF waves is difficult or even impossible, the RF waves transmission of the data is substituted by wire transmission. To do this, each data acquisition apparatus is furnished with a system for automatic switching to one or the other of the functionalities, RF waves or cable.
To transmit the data by RF waves or by cables, it is necessary to implement two different protocols for data transmission depending on whether one or the other of the means of transmission is employed. This necessity is related in particular to the considerable distances which may exist between a data acquisition apparatus and the central system. Moreover, these different protocols use distinct hardware means. This device therefore proposes a solution which is complex and expensive.
Networks for data acquisition by wireless link and in particular wireless networks of seismic sensors using a multi-hop technique must be improved.